It is convenient to describe the background of this idea in relation to the measurement of soil water content but the invention has wider applicability. For example the moisture content sensor may be used in any of the non-exhaustive list of substrates and/or mediums: cotton wool, mineral wool, sand, rock-wool, volcanic material, plant growing media, concrete, building material, pharmaceutical materials and the like. Further, the moisture content sensor may also find application for measuring in the, non-exhaustive, list of applications: environmental monitoring, irrigation monitoring, irrigation control, crop yield optimisation, flood control, damp measurement, building subsidence, refuse compost monitoring and drug manufacturing in the pharmaceutical industry.
Measuring soil moisture content is non-trivial and complex with a number of well known problems in order to measure to any level of accuracy and reproducibility. This is in part due to soil non-homogeneity and variation of soil composition, but also largely due to other environmental and soil conditions. Most crops are grown in soil with a salinity and nutrient level corresponding to an electrical conductivity between 60 mS/m to 400 mS/m (0.6 dS/m to 4 dS/m). This value can be as low as 20 mS/m and as high as 500 mS/m to 600 mS/m for certain crops, such as tomatoes, with soil conductivity in coastal environments up to 3,000 mS/m. With pressure on food production worldwide crops are increasingly being grown in arid and semi-arid regions, many of which have an indigenous salinity problem. At the same time in many areas salinization is on the increase every year because of indiscriminate use of ground water and poor drainage facilities. In many environments temperature can vary noticeably over a 24 hour period, varying as much as 40 degC in some cases. For soil moisture content sensors used in monitoring and control applications there are further application considerations such as moisture content sensor response time and hysteresis in sensor response.
Variation in soil salinity and nutrients can cause errors, which may be significant, in estimated soil water content. To further add to the complexity of accurate soil moisture measurement the conductivity of the soil is temperature dependant and as such if a moisture content sensor has significant salinity response, and there is a temperature change, then the moisture content sensor will be affected. The affect will generally manifest itself as an apparent temperature sensitivity error, resulting from the salinity response of the moisture content sensor.
Dielectric soil moisture content sensors address a number of the application and accuracy issues of other sensing technologies, offering reproducible readings between different moisture content sensors and benefiting from a rapid response time, with no hysteresis in response for rapidly wetting or drying soil. The performance of a dielectric moisture content sensor is however commonly limited by sensitivity to salinity and nutrient levels in the soil, as well as sensitivity to temperature change.
There are a number of known techniques for measuring soil water content. The Neutron probe has been widely used but such apparatus is expensive and since they include radioactive material they increasingly face regulatory burdens including the requirement that they cannot be left unattended. Such expense and regulatory requirements reduce their usefulness for applications such as irrigation. Neutron probes have also been found to be inaccurate in the top 15 cm of the soil. This region is of particular interest for shallow rooted plants and in particular a large number of commercially grown shallow rooted crops.
Matric potential sensors measure soil matric potential rather than volumetric water content of the soil. Soil moisture is transferred to the matric material from the soil and the electrical resistance of the matric material is in turn measured. Matric potential sensors are commonly made from gypsum and are low cost but have slow response times, significant hysteresis, and commonly have a short life-span (particularly in highly acidic soil where they can last less than 1 season). Typically there is significant variance between differing sensors and calibration is complex, with sensors commonly un-calibrated with poor absolute accuracy, and as such are commonly used for monitoring trends rather than absolute soil properties. Slow response time and hysteresis limit a sensors application in certain soils, particularly sandy soils, where a wetting front can rapidly pass through the soil. Similarly this can limit their application in irrigation control where a fast response to soil changes can be needed.
Tensiometers measure soil tension, a measure of how easy it is for plants to take moisture from the soil. Tensiometers are typically water filled and require regular maintenance.
TDR (Time Domain Reflectometry) works by measuring the time propagation of an electromagnetic wave through the soil. This technique can be less accurate in some soils, such as fine particle soils, and can require complex and expensive electronics to implement.
U.S. Pat. No. 5,424,649, discloses a moisture content sensor with probes that have a thin dielectric coating. Such an arrangement desensitises the sensing electronics to an extent from soil conductivity. The electronics used in this moisture content sensor are highly sensitive to soil conductivity, necessitating the use of a dielectric barrier between the sensing electronics and the soil. A dielectric coating is only partially effective at reducing sensitivity to soil conductivity which results in a moisture content sensor with sensitivity to soil conductivity and salinity. The arrangement is such that the dielectric barrier needs to be thin in order to achieve a level of sensitivity (a thickness of 0.05 mm is outlined). This results in probes that are not robust, limiting the applications in which the moisture content sensor may be used reliably.
Another prior art example is U.S. Pat. No. 5,859,536 which discloses a moisture content sensor using two impedance matching networks, with the two networks used to match the impedance of the soil medium to that of an oscillator source impedance. The disclosed arrangement measures a rectified signal on the output side of the matching circuits, directly at the probe inserted in the media. Such an arrangement is found to offer limited insensitivity to the conductance of soil.
Another prior art example is U.S. Pat. No. 5,804,976 which discloses a dielectric moisture content sensor that utilises a probe arrangement inserted in the soil, and a transmission line. The probe arrangement and transmission line are arranged such that the impedance of the soil in the probe arrangement is nominally matched to that of the transmission line. A signal is injected into the soil. If the impedance of the transmission line is mismatched from that of the probe then a proportion of the signal will be reflected. Reflections from the injected signal are used to determine the volumetric water content of the soil. Such arrangement measures two signals, one of which is on the output side of the moisture content sensor and so is still susceptible to variation in the conductance of soil, noticeable at high volumetric water content, which can be a particular issue when used in artificial growing substrates, such as mineral wool or rockwool.
In arrangements that use impedance matching to match the impedance of the soil and measure the signal directly at the probe inserted in the soil or other medium, in the range of conditions commonly found in agricultural soils, it is likely that, even at high frequencies such as 100 MHz, the signal will be significantly influenced by ionic content of the moisture being measured (due to nutrients, salt, fertiliser, etc.), particularly at high volumetric water content, which is undesirable.
The transmission properties of the soil are dependent upon a range of factors including whether there are any impurities within the soil, the temperature of the soil and even the constitution of the soil itself. For example, if there are impurities in the soil then the conductance of the soil may vary significantly and the moisture content sensor should preferably be arranged to be insensitive to such conductance variations. It is therefore a problem to produce a moisture content sensor that provides the water content of soil accurately across a range of conditions that may be experienced.